1. Field of the Invention
The present invention relates to a planetary gear transmission which comprises three planetary gear trains.
2. Description of the Prior Art
Planetary gear transmissions are widely used as automatic transmissions for automobiles or the like. Many conventional planetary gear transmissions comprise two planetary gears such as Ravigneaux gear trains, Simpson gear trains, or the like, which are combined with each other, and generally have gear positions up to a fourth forward gear position. To meet demands for more gear positions for improved running characteristics, there have been proposed transmissions having gear positions up to and more than a fifth forward gear position, and some of those proposed transmissions have already been in use.
Transmissions with an increased number of gear positions are disclosed in Japanese Laid-Open Patent Publication No. 63-318349 and Japanese Laid-Open Utility Model Publication No. 61-103654, for example. The disclosed transmissions comprise two planetary gear trains each combined with three clutches and three brakes, and have six forward gear positions and one reverse gear position. Since only two planetary gear trains are employed, the disclosed transmissions may share components with conventional planetary gear transmissions. However, the disclosed transmissions are complex in transmission control because they cannot avoid gearshifts which require two engaging means (a clutch and a brake) to be disengaged and two other engaging means to be engaged at the same time.
For example, when the disclosed transmissions effect a gearshift from the second gear position to the third gear position or a gearshift from the third gear position to the second gear position, it is necessary for the transmission to disengage one clutch and one brake and also to engage another clutch and another brake.
Japanese Laid-Open Patent Publications Nos. 59-222644 and 1-320362, for example, disclose planetary gear transmissions each having three planetary gear trains. In the disclosed planetary gear transmissions, two elements of each planetary gear train are mechanically coupled to elements of the other planetary gear trains, and four clutches and three brakes are combined with the planetary gear trains. The disclosed planetary gear transmissions have five forward gear positions and one reverse gear position which can be selected by controlling the operation of the engaging means (i.e., the clutches and the brakes). More specifically, any gearshifts between adjacent ones of the five forward gear positions can be accomplished by disengaging one engaging means (a clutch or a brake) and engaging another engaging means. Therefore, controlling the disclosed planetary gear transmissions is relatively simple.
The above planetary gear transmissions with three planetary gear trains are, however, problematic in that in certain gear ranges (e.g., a LOW (1ST) range, a 5TH range, a REV range, or the like), either one of the elements (i.e., a sun gear, a carrier, and a ring gear) of the planetary gear trains rotates at a speed considerably higher than the speed of rotation of the engine (i.e., the input shaft), i.e., rotates at excessive speed. Consequently, the planetary gear transmissions may not be combined with engines that rotate at high speed, although they are suitable for use with engines that rotate at low speed, such as diesel engines.
Transmissions with five gear positions are often required to have a wider range of speed reduction ratios from the first gear position (LOW) to the fifth gear position for better running performance than transmissions with four gear positions. The wider range of speed reduction ratios is also conducive to the rotation of either planetary gear train element at excessive speed.